Refrigerate the metal bowl you will use for mixing for at least 10 minutes. Or chill it in the freezer for five minutes. This will make it easier to whip your frosting.
Make the whipped cream by using a mixer to whip the heavy cream, powdered sugar, lemon juice, and cinnamon in the cold bowl until it forms stiff peaks, about 3-5 minutes. Make sure the peaks are as stiff as you can make them. Refrigerate immediately and set aside for now.
Cut the top and bottom of the watermelon to create even flat surfaces on both ends. The watermelon should be able to stand upright now. Trim off all the skin on the watermelon.
Shape the watermelon into a cake form by trimming the sides into straight lines and cutting more off the top and bottom.
Pat the watermelon dry with a paper towel as thoroughly as you can.
Coat the entire watermelon with previously made whipped cream.
Decorate with raspberries, blueberries, and strawberries. Refrigerate until ready to plate and serve.
PRO-TIP: Cream whips best when it's cold, so we recommend using a chilled bowl to make it. Just pop it in the freezer for about 15 minutes prior to making it!